The present invention relates to apparatus for measuring the turbidity of fluids; and particularly to optical turbidimeters which utilize light emitters and detectors to sense the turbidity.
Turbidity is an optical characteristic of a fluid that is related to the presence, nature and amount of suspended matter or particles which scatter light in an otherwise pure fluid. Turbidity may be sensed by instruments commonly known as turbidimeters which measure the characteristic in terms of the amounts of light which are transmitted directly through and scattered by the fluid.
U.S. Pat. No. 3,775,013 discloses a turbidimeter which utilizes two light sources and two detectors in which each detector is aligned with a different light source. Alternately each light source is energized and the amounts of light detected by the aligned and unaligned detectors are compared. The detector signals produced when each light source is energized are processed to derive a turbidity value, as defined by the U.S. Environmental Protection Agency. Such photoelectric instruments permit turbidity measurements to be conducted on static fluids or those which flow continuously between the emitters and detectors.
The presence of gas bubbles entrained in the fluid affects the optical transmissivity, and therefore the turbidity measurement. As a consequence, gas bubble removal is important to the accuracy of the instrument. Various mechanisms, such as the one described in U.S. Pat. No. 3,849,002, have been proposed to remove air bubbles from the fluid flow before reaching the section of the instrument at which the turbidity is measured.
A further factor affecting instrument accuracy, particularly at low turbidity values, is stray light that is scattered from the various internal surfaces of the measuring cavity. Such stray light produces an erroneous indication of turbidity because it is not caused by light scattered from material suspended in the measuring zone.
Another factor affecting instrument accuracy is the calibration of its circuitry which transforms the detector signals into the turbidity value. Conventional techniques for performing such calibration involved introducing a fluid having a known turbidity into the sensing region. The instrument then was operated while the circuitry was adjusted to produce a turbidity measurement which coincided with the known turbidity of the fluid. The reference fluids must be prepared carefully to insure a uniform, desired turbidity. A commonly used reference fluid called Formazin contains a known carcinogen. In addition, such reference fluids often have a relatively short "shelf life" after which the suspended particles settle or agglomerate resulting in a non-uniform fluid. Thus a certain level of care must be taken during the calibration process to insure that the reference fluid in fact has the known turbidity.